Transient heat transfer on a hot plate cooled by a water spray and an impinging jet

摘要

Experiments were performed to inevstigate the transient fluid flow and heat transfer on a hot plate cooled by a full-cone water spray and an impinging circular jet. A transient flow boiling model for upward impingment is proposed and verified by flow visualization and time-dependent temperature measureemnts. In heat transfer. local heat fluxes and heat transfer coefficients are evaluated using transient surface temperature measurements coupled with a numerical data reduction technique. The results show that the peak heat flux increases as the initial plate temperature increases up to 400 deg C. Comparisons between the water spray and the circular jet are discussed in terms of wetted zone. heat flux and heat transfer coefficient. In boiling heat transfer, the transient peak heat fluxes up to 3.5 x 10~7 W/m~2 are flund at instantaneous surface temperatures in the range of 180-240 deg C. The wetted zone is found to decrease as the initial surface temperature increases. For a surface temperature exceeding 240 deg C only a small wetted zone directly under the jet impinging region is observed; the water is boiled off immediately after contact and depart the surface in the form of conical sprays. The wetted zone is also found to advance with time as the surface is cooled below the Leidenfrost temperature. The circular jet has a higher cooling efficiency than the water spray for an initial surface temperature up to 900 deg C.